Polymerase chain reaction (PCR) is a process for amplifying or multiplying quantities of double-stranded deoxyribonucleic acid (DNA). In a PCR apparatus, a thermal cycler block has one or more wells for holding vials containing a suspension of ingredients for a reaction to produce more DNA starting with “seed” samples of the DNA. The starting ingredients in an aqueous suspension, in addition to the a seed sample, include selected DNA primer strands, DNA elements, enzymes and other chemicals. The temperature of the block is cycled between a lower temperature extension phase of the PCR reaction at about 60° C., which is the phase where all of the DNA strands have recombined into double strands, and a high temperature denaturing phase at about 95° C., during which the DNA is denatured or split into single strands. Such a temperature program essentially doubles the DNA in each cycle, thus providing a method for replicating significant amounts of the DNA from a small starting quantity. The PCR process is taught, for example, in U.S. Pat. No. 4,683,202.
Quantitative measurements have been made on the DNA production during the PCR process, to provide measures of the starting amount and the amount produced. Measurements and computation techniques are taught in U.S. Pat. No. 5,766,889 (Atwood), as well as in an article “Kinetic PCR Analysis: Realtime Monitoring of DNA Amplification Reactions” by Russel Higuchi, et al., Bio/Techinology vol. 11, pp. 1026–1030 (September 1993), and an article “Product Differentiation by Analysis of DNA Melting Curves during the Polymerase Chain Reaction” by Kirk M. Ririe, et al., Analytical Biochemistry vol. 245, pp. 154–160 (1997).
Prior measuring techniques have utilized microvolume fluorometers (spectrofluorometers) and a simple arrangement of a video camera with illumination lamps. Such apparatus utilize dyes that fluoresce in the presence of double-stranded DNA. These techniques and instruments are not particularly adapted to PCR apparatus for routine monitoring of the reaction. There also is a need for greater precision during the monitoring and measurements. Previous instruments that allow real time acquisition and analysis of PCR data have been very basic devices without the required dynamic range, do not have built-in calibration means, do not allow operation with sample well caps, or are very expensive.
An object of the present invention is to provide a novel optical instrument for quantitative monitoring of DNA replication in a PCR apparatus. Other objects are to provide such an instrument with improved dynamic range, automatic selection of exposure time to extend dynamic range, automatic adjustment for drift, simplified operation, relatively low cost, and easy changing of optics to accommodate different fluorescent dyes.